The Antimicrobial and Immunomodulating Actions of Milk Leukocytes

Abstract

The ability of highly developed organisms to defend themselves against invading microorganisms depends on recognizing and destroying dangerous substances such as microbes and their toxins. Recognition and destruction are tasks of the immune system, primarily of the leukocytes. Subpopulations of leukocytes variously release antimicrobial substances such as immunoglobulins, lysozyme, complement factors and reactive oxygen metabolites into the cellular environment upon exposure to an antigenic stimulus, and they also ingest foreign materials or secrete cytokines which regulate the immune response. During lactation a mother can pass immunogenic substances to her offspring through her mammary secretions. These compounds in colostrum and milk are an important, if not decisive, part of the adoptive transfer of immunity from mother to offspring in many species. Most scientific investigations into the adoptive transfer of immunity through milk have focused on immunoglobulins rather than on leukocytes. However, adoptive transfer of the cells in milk also appears to occur. It is important to explain how immunocompetent maternal leukocytes can modulate immune responses in the newborn without recognizing and destroying the neonate’s leukocytes and tissues as nonself elements that bear different, paternally-derived major histocompatibility complex (MHC) antigens. Conversely, why don’t the immunocompetent leukocytes of the newborn attack and eliminate milk-derived cells that bear a maternal MHC? An important purpose of this chapter is to address these intriguing questions to the extent that current data permit. Another important question is whether mammary leukocytes have any protective role or whether the milk is simply a convenient route of excretion for these cells. A full understand-ing of the anti-microbial action of mammary gland leukocytes for all mammalian species must await more exhaustive study.

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